In modem wavelength division multiplexed (WDM) optical transmission systems, there is a need to dynamically equalize the gain of the various data-carrying channels as they pass through the optical network. A large number of factors, including attenuation through the fiber itself, unequal amplification as a function of wavelength as the channels pass through cascaded Erbium Doped Fiber Amplifiers (EDFAs), and others contribute to channel qualities that can degrade the performance and bit-error rate of the system overall. A Dynamic Gain Equalizer (DGE) module equalizes WDM channels or groups of channels to ensure optimal amplification and optical signal-to-noise ratio (OSNR), thus minimizing the bit-error rate (BER) for each channel, while extending transmission distance and expanding usable bandwidth.
Historically, the chief contributor to gain unevenness has been the EDFA. Due to the inherent gain response of the EDFA's operation, there is always a modest imbalance in the gain applied as a function of wavelength. In typical network applications, multiple EDFAs are employed along the total span of the network to boost the signal as it is attenuated through the fiber. As each of the EDFAs imparts a characteristic gain profile to the band, the total unevenness increases in an additive manner. The net result after several EDFAs can be a wholly objectionable power imbalance across the various channels in the band.
In order to compensate for this effect, manufacturers of EDFAs typically insert a static optical element called a Gain Flattening Filter (GFF) into the optical path inside their EDFA modules. A GFF is typically manufactured by depositing a large number of thin films onto a piece of optical glass. The characteristics of the thin films (their thickness and indices of refraction, for example) are carefully selected and controlled during deposition such that they create optical resonances and interferences that effect the transmission of light as a function of wavelength. If properly designed, a GFF can be created in such a way that it completely offsets the effects of the EDFA for a given total input power.
FIGS. 1A-1C illustrate the effect of a GFF attenuation profile on an EDFA gain profile. FIG. 1A illustrates a representation of a gain profile of a typical EDFA. The gain profile indicates how different wavelength signals are attenuated to varying degrees as the signals are impacted by the EDFA. FIG. 1B illustrates an attenuation profile of a typical GFF used to offset the effects of the EDFA imparting the gain profile illustrated in FIG. 1A. Ideally, the attenuation profile of a GFF will be the inverse of the gain profile of a corresponding EDFA. FIG. 1C illustrates the resultant gain of an EDFA with GFF where the EDFA includes the gain profile of FIG. 1A and the GFF includes the attenuation profile of FIG. 1C. A flat resultant gain, as illustrated in FIG. 1C, indicates that the GFF completely offsets the power imbalance effects of the EDFA.
In practice, however, there are a number of factors which render the simple “EDFA plus GFF” formula inadequate. First, while EDFAs have characteristic gain profiles, there can be some manufacturing variability between unit-to-unit and lot-to-lot. The GFFs are even more notoriously difficult to manufacture with consistent performance, due to the large number of different thin films that must be deposited with high repeatability and consistency. Small changes in manufacturing conditions can result in significant changes in performance, making the GFF both expensive and inconsistent. The films on the GFFs can also bleach over their lifetime, rendering them less effective over time. Furthermore, in modern optical networks, where specific optical channels may be frequently dropped or added, there is a need to dynamically effect the gain profile. The profile of the EDFA changes as a result of total power, so as channels are added or dropped, the profile itself changes. A solution that relies wholly upon a static GFF cannot provide adequate flatness to satisfy these changing network requirements.
DGEs have been proposed as a next-generation substitute for GFFs. Because they are variable, they can be configured in the field to optimally flatten a specific set of EDFAs after they are actually powered up. Because they are dynamic, they can respond to changing network conditions as channels are added and dropped.
A number of factors effect the design of the DGE. For example, the DGE should have adequate dynamic range and attenuation slope to flatten the total gain imbalance in the system. Generally, the greater the dynamic range and attenuation slope of the DGE, the greater the number of EDFAs that can be cascaded. As EDFAs are added to lengthen a single optical span, each EDFA adds its characteristic gain imbalance, requiring greater dynamic range and attenuation slope at the DGE for compensation. Thus, there is a rather direct con-elation between the dynamic range and attenuation slope of the DGE and the length of the optical span than can be achieved.
As a practical matter, however, the desire to increase the dynamic range of the DGE can be offset by other factors. For example, it may be more expensive to implement a DGE with wide dynamic range. A DGE that is designed to have a wide dynamic range may induce greater insertion losses when operating in its transparent, or non-attenuation, mode. When operated close to the limit of its dynamic range, a DGE may exhibit degraded performance in terms of polarization dependent losses (PDL), chromatic dispersion or other objectionable effects.
What is needed is a gain equalizer that dynamically attenuates and increases the dynamic range, but does so at a lower cost.
What is needed is a gain equalizer that dynamically attenuates and increases the dynamic range, but does so without significantly increasing deleterious effects such as PDL and insertion loss.